The present invention relates to a micro gas turbine engine provided with an active tip clearance control.
Environmental concerns and increased demands for efficient utilization of available energy resources have prompted the development of various forms of power units. A micro gas turbine engine is one of such power units. Gas turbine engines have been known as a relatively clean and efficient power unit, but their uses have been relatively limited so far because of their size limitations. However, recent development in manufacturing technology has enabled the size of a gas turbine engine to be extremely small.
For instance, by making a rotor shaft incorporated with a compressor wheel and a turbine wheel as an integral ceramic member, it is possible to reduce the length of the gas turbine engine to about 10 cm. When such a micro gas turbine is combined with an alternator or other forms of electric generator, it is possible to replace batteries which have been widely used as small power units but are known to be relatively inefficient, heavy and inconvenient.
Small gas turbine engines are attractive as they can deliver improved power densities with high frequency operation. A major drawback of micro gas turbines is their low efficiency as compared to their large scale counterparts. A key parameter in controlling the efficiency is the clearance between the compressor blades and shroud. It is therefore important to control the tip clearance of the compressor section. Because of manufacturing tolerances and complex modes of thermal expansion of various components, a fixed tip clearance does not allow each particular gas turbine to achieve a satisfactory performance at all times. Therefore, it is desirable to actually measure the tip clearance and utilize the measured tip clearance for the active control of the tip clearance by feedback.
In view of such problems of the prior art, a primary object of the present invention is to provide a micro (miniature) gas turbine engine with an improved tip clearance control.
A second object of the present invention is to provide a micro gas turbine engine with an improved tip clearance control which can operate efficiently under all operating conditions.
A third object of the present invention is to provide a micro gas turbine engine with an improved tip clearance control which can accommodate manufacturing tolerances of component parts and complex modes of thermal expansion of various component parts.
A fourth object of the present invention is to provide a micro gas turbine engine with an improved tip clearance control which is simple in structure and economical to manufacture.
According to the present invention, these and other objects can be accomplished by providing a gas turbine engine with a tip clearance control, comprising: a rotor shaft rotatably supported by a bearing; a compressor wheel integrally joined with the rotor shaft and forming a radial compressor section in cooperation with a surrounding shroud to compress intake air; a combustion chamber for burning fuel by using compressed air produced by the compressor section; a turbine wheel coupled to the rotor shaft and defining a turbine section in cooperation with a surrounding shroud, the turbine section including an inlet end communicating with an outlet of the combustion chamber and an outlet for expelling combustion gas therefrom; a sensor for detecting a tip clearance between the compressor wheel and surrounding shroud; an actuator for selectively causing an axial displacement of the rotor shaft; and a controller for activating the actuator according to an output from the sensor; the sensor comprising a first electrode formed over a surface part of the compressor wheel, and a second electrode formed over a shroud part opposing the first electrode, the controller being adapted to detect a capacitance between the first and second electrodes as a measure of an axial displacement of the rotor shaft.
According to a preferred embodiment of the present invention, the first electrode extends to a surface part of the rotor shaft, the sensor further comprising a third electrode formed over a housing part opposite the extension of the first electrode formed over the rotor shaft, the controller having a first input end connected to the second electrode and a second input end connected to the third electrode.
Because the capacitance is approximately inversely proportional to the size of the tip clearance, a particularly high sensitivity can be achieved in parts where the tip clearance is small, and a highly precise tip control can be achieved. Non-linearity of capacitance as a function of tip clearance can be utilized as an additional signal for tip clearance evaluation to cope with the difficulties in determining absolute capacitance values. The sensor output is given by a serial connection of two capacitors. However, the capacitance between the third electrode and the extension of the first electrode can be made relatively large and relatively invariable with the axial displacement of the rotor shaft so that the capacitor formed by the third electrode and the first electrode extension serves as an electric coupling. Therefore, even though the first electrode is not accessible via a physical lead, it is possible to indirectly detect the capacitance between the first and second electrodes.
Alternatively, a third electrode may be formed over a shroud part adjacent to the second electrode, the controller having a first input end connected to the second electrode and a second input end connected to the third electrode. In this case also, the sensor output is given by a serial connection of two capacitors. The second and third electrodes may be arranged simply one next the other or they may be individually surrounded by guard electrodes to shut off external disturbances or stray capacitance.
Because the profile of the rotor is in most part defined by individual rotor blades, it is important to arrange the second and third electrodes by noting the locations of the rotor blades. For instance, if the second and third electrodes are intended to oppose a common rotor blade, space limitations may create some difficulty in favorably arranging the second and third electrodes on the shroud. Based on such considerations, the second electrode and third electrode may be arranged to be aligned substantially with the edge of one common rotor blade or the second electrode and third electrode may be arranged substantially along a common circumference so as to align with different rotor blades. It would be also possible to offset the second electrode and third electrode from each other both in the axial and circumferential directions so as to oppose different parts of different rotor blades as long as these different parts of the rotor blades pass the two electrodes substantially at the same time.
According to yet another preferred embodiment of the present invention, the sensor comprises a first electrode formed over a surface part of the rotor shaft, a second electrode formed over a housing part opposing the first electrode, a third electrode formed over a housing part opposing the first electrode and adjacent to the second electrode, and a common electrode interposed between the second and third electrodes, the first electrode overlapping only partially with the second and third electrodes so that the electrodes jointly form a differential capacitive assembly, the controller having a pair of differential inputs connected to the second and third electrodes and a common input connected to the common electrode to detect a capacitance between the first and second electrodes as a measure of an axial displacement of the rotor shaft.
Thus, a differential capacitance measurement gives a linear measure of the axial displacement of the rotor shaft or the tip clearance, and this allows a favorable feedback control of the tip clearance by using the actuator.
A capacitance between two electrodes is determined not only by the distance between the two electrodes but also by the dielectric constant of the material interposed between the two electrodes. Therefore, if a pair of electrodes are provided on the side of the shroud, and the compressor blades pass through the electric field formed between the two electrodes, the tip clearance as well as the presence of the blades can be determined as a change in the capacitance between the two electrodes, and no electrode is required to be formed on the compressor blades. To contain the electric field within a prescribed area and avoid external disturbances, the two electrodes may be formed in such a manner that one of them is surrounded by the other.